The "red eye" effect in photographic images occurs when a person is photographed with a flash in a low light environment. Under such conditions, the person's pupil becomes dilated, allowing the bright light of the flash to penetrate the pupil. The photographic image of the eye can then include the image of the actual retina of the person. Since the retina is red, the image of the retina in a photographic image appears as a red area defined by the pupil and is referred to as `red-eye" effect.
One method of correcting for red-eye effect in photographs is to use specialized pens to paint over the red-eye affected pupil image with dark ink. The ink is typically permanent. Another method of correcting for red-eye effect in photographs is to scan the image into a computer and recolor the red-eye affected image using a photo editing software.